Spin-Charge Conversion in Sb2Te3-based heterostructures

Spin-charge interconversion (SCIC) refers to the conversion of pure charge currents into pure spin currents and vice versa. These fundamental phenomena can be exploited to generate and manipulate electronic states for use in spin-based logic and memory devices. A wide variety of materials with high spin-orbit coupling, including topological insulators (TIs), are emerging as promising candidates for the implementation of SCIC devices. Although TIs are insulating in the bulk, they exhibit spin-polarized conductive electronic states localized on their surfaces. These states, known as topological surface states (TSSs), exhibit spin-momentum locking, meaning their spin and momentum directions are mutually orthogonal in reciprocal space. This property of TIs makes them particularly attractive for the design of efficient SCIC devices. TSSs are robust against nonmagnetic disorders, such as defects and impurities, but are disrupted by the proximity of magnetic materials, which are essential parts of SCIC device. To practically utilize TI in devices, it is necessary to devise a method to separate the TI from the magnetic material while keeping the TSS properties unaffected.

In this study, we explored Au and Al metals, which are widely used in the electronics industry, as spacer materials between a TI (Sb₂Te₃) and a magnetic (Co) film. Both materials have large spin diffusion lengths and, therefore, are suitable for supporting spin-polarized currents. Spin-pumped ferromagnetic resonance imaging (SP-FMR) studies showed that a high rate of spin-to-charge conversion is detected in a device based on the Sb₂Te₃/Au/Co heterostructure (Fig. 1a), while no conversion is observed when considering the Sb₂Te₃/Al/Co heterostructure (Fig. 1b). 

Figure 1: (a) Spin pumping measurements show efficient spin-charge conversion for the Sb₂Te₃/Au/Co heterostructure. (b) The conversion is suppressed in the Sb₂Te₃/Al/Co heterostructure.

To understand this difference in functional behavior, we analyzed the electronic and chemical properties of the interface between the TI and the Au and Al layers by performing angle-resolved photoemission spectroscopy (ARPES) experiments and core-level analysis at the high-resolution VUV-Photoemission beamline of Elettra. Au and Al were thermally evaporated under ultrahigh vacuum conditions onto in-situ exfoliated Sb₂Te₃ single crystals. 

The ARPES spectra demonstrate that the Sb₂Te₃ TSS is still observable after the deposition of a 0.9 nm thick Au film (Fig. 2a) and retains the same energy-momentum dispersion as the clean TI surface, albeit with reduced intensity. In contrast, after the deposition of 1 nm Al a clear qualitative change of the TSS dispersion is observed (Fig. 2c). We found that the origin of this behavior lies in the different chemical reactivity of Au and Al with the TI surface. Core-level photoemission spectroscopy shows that the Sb₂Te₃/Au interface is sharp, with a Te 4d doublet remaining almost unchanged compared to that of the freshly exfoliated substrate (see Fig. 2b). By contrast, after Al evaporation, the same core level peak exhibits a notable change in shape, with the appearance of an additional component on the high binding energy side indicative of Al-Te alloy formation (Fig. 2d). 

Figure 2: a, b) ARPES and Te 4d core level spectrum of Sb₂Te₃ after deposition of 1 nm of Au. c, d) ARPES and Te 4d core level spectrum of Sb₂Te₃ after deposition of 0.9 nm of Al.

Our results highlight the importance of choosing a suitable non-magnetic interlayer to preserve the TSS, as well as the influence of material chemistry on SCIC phenomena, highlighting the key role of the TSS in spin-charge conversion processes in Sb²Te³ TI-based devices.

This research was conducted by the following research team:

E. Longo¹, M. Belli², C. Wiemer³, A. Lamperti³, A. V. Matetskiy⁴, P. M. Shevedyaeva⁴, P. Moras⁴, M. Fanciulli⁵, and R. Mantovan³

¹ Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra, Catalonia, Spain
² CNR-IMEM Unit of Parma, Parco area delle Scienze, Parma, Italy
³ CNR-IMM, Unit of Agrate Brianza, Agrate Brianza, Italy
⁴ CNR-ISM Unit of Trieste, Trieste, Italy
⁵ Department of Material Science, University of Milano Bicocca, Milano, Italy

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